This is the continuation of a long-term project to study the ecological impact on arbovirus cycles of drastic environmental changes caused by a construction of a large hydroelectric dam in a tropical deciduous forest area of Panama. The impoundment of the Bayano River will cause the formation of a 300 km2 lake. A study site was selected on a ridge that will become a large island after the flooding. Baseline microclimatic and biomedical data have been gathered during the first 3 years and plans have been drawn to study the changes on populations of vectors and vertebrate hosts of arboviruses after the formation of the island. Up to now the following arboviruses have been isolated from the study site: MAY; EEE; YF; SLE; Sindbu-group (unidentified as yet); Sandfly fever group (unidentified as yet). Ancillary projects to the main program have been developed on such arthropod-borne diseases as Chagas' disease and leishmaniasis also found in the study site. A second aspect of the program is to study vector competence of colonized Culex (Melanoconion) mosquitoes and various serotypes of VE virus and of Haemagogus and Sabethes mosquitoes with low-passaged YF virus. Bibliographic references: Adames, A.J. and Galindo, P. 1975. Description of the immature stages of Galindomyia leei, Stone and Barreto, 1969. Mosquito Systematics (In Press, to be published in the June issue); Adames, A.J. and Arzube R., M.E. 1975. Geographical extension of Galindomyia leei Stone and Barreto to Ecuador (Diptera, Culicidae). Mosquito Systematics. (In Press, to be published in the June issue).